Two principal types of strain gauges have been known. One type is referred to as a wire type gauge and comprises a serpentine pattern of electrically resistive wire, e.g., nichrome wire with a diameter of about 10 .mu.m, bonded to a sheet of paper or felt, the second type of strain gauge, the foil type gauge, comprises a foil of Cu-Ni alloy bonded to a polymer film made of polyimide, polyester, or phenolic resin. This foil normally has a thickness of a few micrometers and is etched with a sinuous pattern on a width of several tens of micrometers. The foil type gauges usually conduct a larger current flow than the wire type gauges.
These types of conventional strain gauges have a gauge factor range of 2 to 3 and must be formed in complex patterns in order to have high sensitivity. The formation of the complex patterns causes each type of strain gauges to be relatively expensive.
The reliability of these types of conventional strain gauges has been low because of problems in bonding thin wires or foils to the substrate with an adhesive.
A third type of strain gauge using a semiconductor device instead of metal wires and foils is also available. Typically, semiconductor strain gauges feature a gauge factor that is 10 to 50 times higher than metallic strain gauges. The brittleness of the semiconductor gauges and difficulty in effecting consistently acceptable bonds with thin semiconductor devices have plagued this type of strain gauge. Also, since changes in the resistance of a strain gauge varies with changes in the angle between the gauge length and the direction of the strain, many gauges must be arranged in predetermined directions in order to measure strain distribution on a large area. This has been difficult to accomplish with conventional semiconductor strain gauges.
Also, it is common to require information concerning the distribution of mechanical forces over a surface. As an example, robot "hands" must be capable of handling various types of objects with forces that are suited to the objects being handled. This requires determining the planar distribution of gripping forces by the robot hands.
A conventional mechanism for measuring the planar distribution is schematically shown in FIG. 1. Perpendicular conductor wires 1', 2' are insulated from each other and are arranged in a lattice form. A plurality of strain gauges 3' are provided in a matrix such that one end of each strain gauge is in contact with one of the wire 1' and the other end of the strain gauge 3' is connected to a wire 2'. A voltage is selectively applied to a pair of terminals 4', 5' connected to the wires 1', 2', respectively, to scan the matrix of strain gauges 3' to determine the distribution of strain in a plane by measuring the changes in the resistances of the gauges in terms of the current flowing through each gauge. For example, the resistance of the gauge 31' can be measured by applying a voltage between the terminals 41' and 51'.
In order to obtain accurate measurements, each strain gauge 3 must be provided with a series-connected blocking diode to limit current flowing into the gauge of interest from other gauges. The resistance of each strain gauge 3 is dependent on the angle that the gauge length forms with respect to the direction of the strain. Therefore, to determine the distribution of strain accurately, the strain gauges 3 must be arranged in predetermined directions and as many strain gauges 3 as possible should be used to obtain precise measurements of strain distribution. It is, however, very difficult to bond a large number of strain gauges in exactly the same direction. Considerable difficulty is also involved in connecting a blocking diode to each strain gauge without causing undesired changes in the measurements of strain distribution.